The invention relates to a printing unit for typical use in a price-labelling device or the like.
In such devices, it is possible for the printing types to be set at will into the printing lines, with the aid of setting wheels and a setting shaft.
In certain known printing devices, each setting wheel has guided around it an endless type band, on the external side of which are arranged printing types and possibly also reading types associated with the printing types, while driving projections are arranged on the inside of the said band and these projections engage in tooth gaps of the setting wheel. It is also known to provide the setting wheels on their circumference with printing types and possibly also reading types, so that the setting wheels simultaneously also form printing type wheels.
It is also known to arrange the detent recesses on the setting shaft. The setting shaft is provided with tangentially encircling grooves corresponding to the number of the setting wheels, which grooves have spacings from one another which correspond to the widths of the setting wheels and in which engages a resilient detent member, which is arranged in a fixed position on the housing of the printing unit and is formed by a ball, which is guided in a bore of the housing and which is under the action of a small helical spring.
This known detent or latching device has the disadvantage that the manufacture thereof is costly and that the fitting thereof is complicated and time-consuming. Furthermore, this printing unit has the disadvantage that, with printing units of equal width but a different arrangement of wider or narrower printing type bands or printing type wheels, it is necessary to have setting shafts which are adapted to the arrangement of the wider or narrower bands or wheels.
These disadvantages are at least partially avoided by other known printing devices. For example, in one known device, the printing type wheels provided with the appropriate types have on their periphery recesses which are engaged by projections or teeth of the setting wheels. So as to produce the axial latching of the setting shaft, the latter has fixed thereon so as to be axially immovable a pointer which engages over the circumference of the setting wheels, the said pointer comprising at its free end a detent member projecting at right-angles to the setting shaft and the said detent engages in the latching recesses of an easily replaceable detent rail. By replacing the said detent rail, the said printing unit is capable of being adapted to the actual arrangement of printing type wheels or printing type bands of different widths.
A disadvantage as regards this printing unit is that the detent member and the detent recesses co-operating therewith are spaced from the setting shaft, so that bending moments have to be transmitted by the pointer. As a consequence, the pointer has to be constructed and guided in a suitable manner for absorbing loads.
In another known device, the detent recesses are formed by bevelled surfaces on the edges, which are arranged on the bosses of the setting wheels. The bevelled edge surfaces of two setting wheels arranged in juxtaposition form an encircling groove, which is engaged by resilient detent members arranged on the setting shaft. This printing unit has the advantage that the setting wheels themselves form tangentially encircling grooves, of which the axial spacings correspond to the respective widths of the setting wheels and of the printing type bands which are guided over said wheels. However, this printing unit has the disadvantage that the grooves formed by edge bevelled surfaces can only be of relatively small cross-section and these grooves are interrupted by the internal toothing of the setting wheels, which toothing co-operates with the driver element of the setting shaft. The consequence thereof is that resilient detent members are necessary for "feeling" the grooves, the width of said detent members being narrower than the tooth division of the internal toothing. In order to produce an adequate detent or latching effect, it is necessary with the known printing unit to have at least two resilient detent members on the setting shaft. Because of the small size of the grooves and of the resilient detent or latching members and because of the necessary clearance between the individual setting wheels and between the setting shaft and its sliding movement in one wall of the housing and also between the setting shaft and the setting wheels, on the other hand, only narrow production tolerances are permissible with the known printing unit, if the detent or latching mechanism is to be reliable in operation. It is for these reasons that the manufacture of this known printing mechanism or unit is expensive.